(a) Field of the Invention
This invention relates to a process for producing an aromatic polyamide from an aromatic diisocyanate and an aromatic dicarboxylic acid. More specifically, the present invention is concerned with a process for obtaining an aromatic polyamide, which is colored little and has a high polymerization degree, by using a particular reaction solvent.
(b) Description of the Invention
An aromatic polyamide produced by using an aromatic diisocyanate and an aromatic dicarboxylic acid as raw materials is an industrially useful material. A variety of production processes have therefore been disclosed. Examples known include to use as an aprotic polar solvent a linear or cyclic amide or phosphoryl amide such as N,N'-dimethylethyleneurea, N,N-dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyrrolidone, .gamma.-butyrolactone or hexamethylphosphoric acid triamide, or a sulfoxide or sulfone such as tetramethylenesulfone, diphenylsulfone or dimethylsulfoxide and also to employ an alkali metal salt of a polycarboxylic acid as a catalyst (Japanese Patent Application Laid-Open No. 151615/1982); to use an alkali metal carbonate or alkali metal hydrogencarbonate as a catalyst (Japanese Patent Application Laid-Open No. 13629/1983); to use an alkali metal hydroxide as a catalyst (Japanese Patent Application Laid-Open No. 67723/1983); or to use an alkali metal compound as a catalyst and also to employ an N,N'-dimethylalkylene-urea as a solvent (Japanese Patent Application Laid-Open Nos. 190517/1986 and 204218/1983).
In the above production processes, the aromatic diisocyanate as one of the raw materials reacts with water contained in a trace amount in the reaction system causing deleterious effects on the molecular weight and physical properties of the resulting polymer. It has hence been common to use the solvent, aromatic dicarboxylic acid, catalyst, etc. after sufficiently drying them to reduce their water contents to their respective predetermined levels or lower. It however takes a long time to dry such aromatic dicarboxylic acids and catalysts in general and, in fact, it is difficult to dry them sufficiently. Among these materials, the alkali metal compound as a catalyst is hygroscopic and is therefore particularly troublesome upon its drying. Drying by microwave heating or the like has thus been attempted.
As has been mentioned above, drying of the raw materials and catalyst needed complex steps and, moreover, some problems were involved in their storage and control after their drying.
Even when solution polymerization was conducted as described above using materials dried separately, the molecular weight of the resulting polymer did not increase. It was therefore difficult to obtain a polymer usable for the production of fibers having high tenacity
Various causes of these problems may be deduced. For example, water, which is a byproduct from the formation of a carboxylic acid salt from the aromatic dicarboxylic acid as a raw material and the alkali metal hydroxide or alkaline earth metal hydroxide as a catalyst, and water contained in a trace amount in the catalyst do not spread into the solvent. Therefore, they probably give certain deleterious effects to the aromatic diisocyanate which reacts on the surface of the catalyst